it's too early in set_delivered() because set_closed() calls
set_delivered() on a nexus when all children are already devivered, but
that can happen from within the nexus node's fsm.

we will add extra finis when it's safe in a later commit

in the varnish-cache variant of the code this works because not VDP_Push
is used but the internal push to the parent

When subreqs still used the topreq's VDPs, we could not do this, but now
that we properly layer the VDPs again, we can (and should)

waiting for the boc happened at the wrong place

***  v1   debug|Assert error in vped_gzgz_init(), foreign/cache_esi_deliver.c line 163:
***  v1   debug|  Condition(foo->start > 0 && foo->start < foo->olen * 8) not true.

... because of T_FINAL.

Because the parent's thread (except for the root) will terminate
eventually, this is also all good, except for the case that we spend all
available threads on T_FINAL.

Leave the hard problems for later, *sigh*

for esi_level = 0, we still need to close the root in the vdp, but for
levels down, we want to make sure that the vdps are right before
anything is happening below our node

the producer should have the T_FINAL lock before making the node
available by releasing the tree lock

those VSL(SLT_Debug, 0, ...) trigger false positives while testing

or at least I fail to see how it should with just one req->vdc

...why I could have meant by topa-rent

This restores src/foreign/cache_esi_deliver.c to basically the code from
varnish-cache and avoids any unlayered pushes / crc.

Never mind that this currently never happens anyway.

in for nexus nodes (= ESI subrequests), we fini the pesi vdp (and,
conceptually, the buf also, but that requires details still) when we are
done with parsing, such that any other vdps are still in place.

Also we push a vdp which pushes bytes up one parent like ved_ved in
varnish-cache.

So we should now have a VDP chain for each subreq basically matching
that of varnish-cache

I always got confused about this myself, so the name was probably bad

gzip status needs to be per esi inlcude (= per nexus):

If the esi is gzipped and is used in an ungzip context, the ungzip vdp
gets pushed by varnish-cache, yet still all content below needs to be
(pretend-)gzipped for that ungzip to work.

geoffs load tests have shown that my previous idea of trying to fool
varnish-cache code in temporarily changing the objcore flags was bad and
leads to all kinds of nasty races.

So so we add yet another method which not only keeps the subreq intact,
but also halts its thread until deliery is ready.

This requires another thread to exist for longer, hopyfully will get us
towards the goals described in d9c36c7e.

We called pesi_finish() after notifying the topreq, so they could both
be running pesi_finish(), breaking out has_task assertion

firstly, this is a cleanup

The relevant fix is that we raced a backend fetches objcore->flags
modification, which simply was wrong. Fiddling the flags under the
oh->mtx should solve this race.

and add back assertions that we hopefully do not mess it up

unsigned is always >= 0.